One-Piece Composite Wing Built for Perpetual-Flight Plane

Researchers at Lehigh University have built an unusual wing, made of a single piece of carbon fiber composite, for an unusual type of airplane: An unmanned craft designed for high-altitude perpetual flight. The 6.5m (21.3ft) wing has no fasteners or adhesive joints.

An unusual wing, made in a single piece from carbon fiber composite, has been built for an unmanned aircraft designed for high-altitude perpetual flight. Shown here with mechanical engineer Joachim Grenestedt, the 6.5m (21.3ft) wing has no fasteners or adhesive joints.
(Source: Lehigh University)

The wing, including wing planks, six internal webs for bearing shear loads, spar caps, and a trailing edge for accommodating wing flaps and ailerons used for flight control, was built in a single molding process. The single-piece construction is highly unusual for composite structures, which are being used increasingly in aircraft, as well as in satellite launchers and spacecraft.

"Usually a structure like this is made in many parts, which then have to be trimmed and joined in advanced fixtures, adding complexity and weight and reducing strength," said Grenestedt in a student newspaper article. "Making complex parts, like a wing, in a single shot is the holy grail of composites manufacturing."

The structure was made from 0.6mm-thick layers of epoxy matrix, reinforced with carbon fibers, in molds that were digitally designed and machined at the university. The material is similar to the composite Grenestedt used in building the Numerette, a 29-ft speedboat with a hull formed of composite sandwich panels bonded to a steel frame.

The wing's top skin was layered into the mold first. Flexible tubing and expanded polystyrene were then used as temporary placeholders to geometrically align the structure for the placement of the internal webbing of carbon fibers. The bottom wing skin was then layered in place, and the structure cured at 121C (250F), which burned away the temporary structure's materials.

An unmanned aircraft that will fly at high altitude, generating its own power from the sun and the jet stream, is the wing's ultimate destination. The project's long-term goal is perpetual flight.

An aircraft designed to fly for years at a time must have long, slender wings, like those on manned gliders, as well as very low drag. It must also fly very fast, so the wings have to be stiff to avoid flutter and divergence, which become critical at high speeds. Finally, it must be strong enough to conduct hard turns at high speed. The new wing was designed to withstand hard turns of up to 20 Gs before failure.

Grenestedt and a team of students fabricated the prototype in about two weeks, and are now testing its ability to withstand extreme forces. The plane's potential applications include launching communications satellites, weather monitoring, and surveillance. The multi-year project is funded by the National Science Foundation and Lehigh University.

William, as I understand it the higher speeds associated with perpetual flight are to keep momentum above certain thresholds so the plane doesn't fall out of the sky. You can find several articles (full text) that go into the subject in greater detail here:http://www.dynamicsoaring.lehigh.edu/wiki/index.php/Publications

The one-piece wing assembly is quite an unusual approach, and the reasons for the choice are certainly important. I had not realized that the multi-piece building up approach added much weight, but I can see where it would have to add weight and volume. The method of fabrication sounds quite traditional, except that many structures use the hexcell type of material for additional stiffness.

The one assertion that I did not understand is the claim that a perpetual flight craft would need to fly very fast. IT would seem that flying at the minimum speed to maintain the desired altitude would require less energy because of less drag losses. For fans, at lleast, it seems like the required power goes up as the fourth power of speed, and so I would expect the drag of a wing to increase with speed also. But that is an area that I have not studied.

It's nice to hear about this collaborative design effort. It would be interesting to follow this story through to the actualy finished "perpetual flight" aircraft. Sometimes innovative fabrication techniques find their way into unexpected places. I like the reference to having used this same technique for a marine craft, for example.

This design removes two critical issues of flight, fasteners and metal/composite fatigue. As this is a perpetual flight aircraft, the flexing of the wings upon takeoff and landing is eliminated. B-52 bombers flex about 6-8 feet at the wingtips during their cycles, leading to frequent inspections and replacements.

Lou, thanks for that input on fastener issues: their absence was one of the unusual aspects of this design that piqued my interest in writing about it. And I agree, I thought it was totally cool that the team combines a ME with a CS. In the student newspaper article, the ME Grenestedt is quoted as saying that his partner, Spletzer, "handles the intelligence aspect of the wing and aircraft," which includes controls and flight trajectories.

Nadine, I agree, it would be great if these advancements could be translated to commercial manned aircraft. But in general, the structure design is not as similar as you might think, since manned aircraft usually carry a lot more weight than unmanned, among other factors. The wing described in this article is designed for an unmanned perpetual flight plane, more like a glider than a Boeing commercial jet, and its design has challenges not present in commercial aircraft design. The links at the end of this article can guide you to more articles we've done about composite use in manned craft, since we write about both.

Very good question, Beth. It wasn't mentioned, and it most likely would have been if they'd used it in any way. The reference to layering is to the typical epoxy composite process, where sheets of fiber are laid down in the epoxy matrix.

Ann, this is indeed a great way to proceed. You mention composited in spacecraft. I worked on one, a long time ago, that had composite tubular frame members and titanium hubs. The first design for attaching the tubes to the hubs had the holes aligned. This produced stress cracks and had to be modified to offset the holes. By eliminating the fasteners, and the different materials, I am sure this structure would be superior. We had CAD and CAE back then, but it was very primitive compared to what we do now.

I was also interested in the fact that the researchers included a Mechanical Engineer and a Computer Scientist, who is actually named. Computer Scientists are a necessary part of many projects these days. Especially when they research projects. They often play the role of the mathematician, solving complex numerical problems that have not been solved before.

There is a lot of technical advancement in aviation focused on unmanned aircraft. Can this be translated for larger planes? Maybe it's just the focus here on Design News. Seeing more advancement in "manned" aircraft would be great too.

The one-piece composite structure seems like a pretty big deal for wing design as the fasterners and adhesive joints usually seem to be the areas that are magnets for potential trouble. Question: Was 3D printing involved in any way?

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